Biological signals are difficult to record when measured non-invasively from a body surface because the amplitude of the biological signals are low in relationship to the amplitude of ambient noise signals. Potential noise sources that can obscure measurement of biological signals from the body surface include broadcast electromagnetic radiation from electric or electronic devices, scattered electromagnetic radiation from neutral sources moving through static fields, mechanical vibrations in the environment transferring to the source and movement of the source itself, among others.
The impact of noise sources on biological signal recording can be minimized by electromechanically isolating a subject from potential interferences using electrical shielding and vibrational isolation. However, in real world applications, such control measures are not feasible and low signal recordings must be made in high noise environments. Further, the power spectrums of real world noise sources often overlap the power spectrums of the biological signal and as such are not amendable to conventional filtering techniques, such as bandpass filtering.